1. Field of the Invention
This invention relates to optical fibre cables, and especially to such cables to be used in underwater applications.
2. Description of Related Art
Underwater cables are required to have high strength and be resistant to high pressures, the action of the sea and damage from vessels. Conventionally, such cables are constructed with a fibre package protected by a seamless metal tube and layers of armouring wires. The metallic elements are insulated and protected by a thick polythene sheath. Where repeaters are needed, the power to the repeaters is supplied from the metallic elements. If the polythene sheath is damaged, there is a risk of the metallic elements being earthed. Thus, in order to provide satisfactory insulation of the wires, a thick sheath is used to encase the cable. The combined effect of the armouring wires and sheath is to produce a heavy and unwieldy cable.
It is an object of this invention to provide a cable which is smaller and lighter than conventional cables, whilst ensuring that there is no additional risk of earthing the metallic elements or of damage to the optical fibres. a central cylindrical core, a layer of strength members at least one of which is replaced in the same layer by a laser-welded metallic tube within which is encased at least one optical fibre, the layer overlaying the surface of the core, and sheathing means overlaying the first layer of strength members.
The example described in said U.K. application is illustrated in FIG. 1 of the instant drawings. Briefly, a central copper conductor 1 is surrounded by a layer of low density polythene which constitutes an inner sheath 3. A first layer of high tensile steel wires 6 is wound helically about the inner sheath 3, a number of optical fibres or fibre ribbons 5 encased in laser-welded stainless steel tubes 7 being interspersed between the wires 6. The steel tubes 7 and the wires 6 have the same diameter.
A second layer of high tensile steel wires 11 is wound helically around the first layer in the opposite direction with a quantity of silicone water blocking compound 9 between the layers. The cable is encased in an outer sheath 13 formed of medium density polythene.
The stainless steel tubes 7 protect the respective optical fibres 5 from damage; each tube replaces an armouring wire. Thus there is no need for a separate optical fibre package within the cable, enabling a lighter and more compact cable to be produced.
Typically, stainless steel tubes of 1 mm to 6 mm diameter are used. These are filled with thixotropic grease and laser welded longitudinally to enclose the optical fibre. The tubes provide moderately hard, hermetically sealed fibre packages.
The lay length of the helical first layer may be varied depending on the fibre strain relief required in the cable.
It is necessary to minimise torsional resistance in the cable, for ease of coiling the cable. Torsional effects in the core can be compensated for by an appropriate selection of diameter and lay length and lay direction of the helical strength member layer or layers but render the torsional behaviour especially susceptible to any change in the number of strength members employed. Cable designs can be required with different tube counts, so it is desirable to have the freedom to vary the number of tubes replacing strength members in a layer, without affecting torsional performance significantly.
The core of the cable can be made relatively soft in order to minimise its torsional resistance, but it then fails to provide sufficient radial reaction by way of support to the strength member layer or layers.